Ionization-induced switch in aromatic molecule–nonpolar ligand recognition: Acidity of 1-naphthol+ (1-Np+) rotamers probed by IR spectra of 1-Np+–Ln complexes (L = Ar/N2, n ≤ 5)

The interaction of the trans (t) and cis (c) rotamers of the 1-naphthol cation (1-C10H8O+ = 1-Np+ = 1-hydroxynaphthalene+) with nonpolar ligands in the ground electronic state is characterized by IR photodissociation spectra of isolated 1-Np+–Ln complexes (L = Ar/N2) and density functional calculations at the UB3LYP/6-311G(2df,2pd) level.

Size-dependent frequency shifts of the O–H stretch vibration (Δν1) and photofragmentation branching ratios provide information about the stepwise microsolvation of both 1-Np+ rotamers in a nonpolar hydrophobic environment, including the formation of structural isomers, the competition between H-bonding and π-bonding, the estimation of ligand binding energies, and the acidity of t/c-1-Np+. t-1-Np+ is predicted to be more stable than c-1-Np+ by 9 kJ mol−1, with an isomerization barrier of 38 kJ mol−1.

The OH group in t-1-Np+ is slightly more acidic than in c-1-Np+ leading to stronger intermolecular H-bonds.

Both 1-Np+ rotamers are considerably less acidic than the phenol cation because of enhanced charge delocalization.

The 1-Np+−Ar spectrum displays ν1 bands of the more stable H-bound and the less stable π-bound t-1-Np+–Ar isomers.

Only the more stable H-bound dimers are identified for t/c-1-Np+–L2.

Analysis of the Δν1 shifts of the H-bound dimers yields a first experimental estimate for the proton affinity of the t-1-naphthoxy radical (∼908 ± 30 kJ mol−1).

The Δν1 shifts of 1-Np+–Ln (n ≤ 2 for Ar, n ≤ 5 for N2) suggest that the preferred microsolvation path begins with the formation of H-bound 1-Np+–L, which is further solvated by (n−1) π-bound ligands.

Ionization of 1-Np−Ln drastically changes the topology of the intermolecular interaction potential and thus the preferred aromatic substrate–nonpolar ligand recognition pattern.


Hydroxyarenes are fundamental molecules in chemistry, biology, and life sciences, and display a variety of interesting properties arising from both the hydroxy group and the aromatic π-electron system.1,2

In particular, they are strong photoacids, that is the acidity of the OH group increases drastically upon electronic excitation and ionization, which eventually may promote proton transfer to a suitable solvent.2–4

The properties of such proton transfer processes, which are among the most fundamental reactions occurring in chemical and biological systems, depend sensitively on the environment.

Consequently, (micro)solvated hydroxyarenes have become popular model systems to investigate energetics and dynamics of photo-induced proton transfer in both isolated complexes and in solution under controlled solvation conditions.2–20

The present work reports quantum chemical calculations and infrared (IR) spectra of 1-naphthol cations (1-Np+ = 1-C10H8O+ = 1-hydroxynaphthalene+) solvated by a well-defined number of nonpolar ligands (L = Ar/N2), providing direct access to the acidity of the OH group in the cation ground state and its dependence on stepwise solvation in a hydrophobic environment.

Significantly, aromatic molecules (A) with acidic functional YHk groups (e.g., Y = N, O) offer two major recognition sites for inert ligands L. L can bind to the aromatic π-electron system (π-bond) or to one of the k acidic protons of YHk (H-bond).

Alternative binding sites, such as H-bonds to aliphatic or aromatic CH protons, are usually less stable.

The preferred binding site in A–L dimers depends on several factors, including the degree of electronic excitation and charge state of A, the acidity of the YHk group, and the ligand type (polar or nonpolar).

For example, complexes of neutral A with rare gas atoms have π-bound equilibrium structures in the singlet electronic ground state (S0).

Examples include Ar complexes of benzene (Bz–Ar),21 phenol (Ph–Ar, YHk = OH),22,23 aniline (An–Ar, YHk = NH2),24 and indole (In–Ar, YHk = NH).25

The major contributions to the A–Ar attraction are dispersion forces between Ar and the π-electron system of A, which favor π-bonding.

In fact, H-bound isomers have not been detected for any aromatic A–Ar complex in S0.

The situation changes for A–N2 dimers, because the quadrupole moment of N2 leads to additional electrostatic interactions with the polar YHk group of A, so that the H-bond may energetically compete with the π-bond.

For example, N2 favors H-bonding to Ph22,26 but π-bonding to less acidic An27 as well as Bz.28,29

In general, the topology of the interaction potential of A+–L radical cation dimers differs qualitatively from that of neutral A–L, because of the significant additional electrostatic and inductive attraction arising from the positive charge.8,13,15,30,31

As a consequence, neutral and ionic A(+)–L dimers often possess rather different equilibrium structures and interaction energies.

For example, Ph+–Ar,32–35 An+–Ar,36 and In+–Ar37 have H-bound global minima in the ground electronic state of the cation (D0), whereas the π-bound isomers are only local minima.

Similarly, Ph+–N2,22,32,33,38 An+–N2,39 and In+–N237 prefer H-bonds over π-bonds in D0.

Because of the additional charge–quadrupole interaction, the H/π-bonds in A+–N2 are stronger than in A+–Ar.

Similar to A+–Ar/N2, the Ar and N2 dimers of protonated aromatic molecules (AH+) with acidic YHk groups (e.g., PhH+–Ar/N2)40–42 also favor H-bonds over π-bonds.

In contrast, the most stable A(H)+–Ar/N2 dimers of A(H)+ ions without YHk groups, such as Bz(H)+–Ar/N2,21,43–45 feature π-bonds, because CH protons are only weakly acidic even in the cation ground state.44

The ionization-induced switch in the preferred ligand binding site in A(+)–L imposes important consequences for spectroscopic studies of A+–L prepared by photoionization of A–L.

Often, A+–L is generated by resonance enhanced multiphoton ionization (REMPI) of A–L formed in a supersonic expansion.

Spectroscopic information of A+–L may then be extracted, for example, from photoionization efficiency spectra (PIE),46 mass analyzed threshold ionization (MATI),21,47 zero kinetic energy photoelectron (ZEKE) spectra,13 or IR photodissociation.8,10

However, all these ionization techniques suffer from the Franck–Condon principle, which prevents significant population of the most stable A+–L isomer for dimers with significantly different neutral and ionic global minimum structures.

This situation is, however, quite common for A(+)–L dimers with both polar and nonpolar L.31,32,48,49

To avoid the limitations of REMPI cluster ion generation, in the present work cold A+–Ln complexes are produced in a supersonic plasma expansion created by electron ionization (EI) of a molecular beam.50

This EI ion source generates A+–Ln complexes via EI of A followed by cluster aggregation.

Consequently, the EI source produces predominantly the most stable isomer of a given A+–Ln complex, independent of the most stable structure of neutral A–Ln.

This is in contrast to the REMPI ion source, which often generates only local minima of A+–Ln.

For example, EI-IR spectra of Ph+–Arn,32–35 An+–Arn,36 and In+–Arn37 were interpreted with global minimum structures, which completely escaped detection in the corresponding REMPI-IR and other photoionization spectra (PIE, ZEKE, MATI).

1-Np is one out of the two possible bicyclic monohydroxyarenes (1/2-Np).

It has two planar isomers, which differ by the orientation of the OH group with respect to the aromatic ring (Figs. 1 and 2), namely the trans (anti) and cis (syn) rotamers denoted t-1-Np and c-1-Np, respectively.

Spectroscopic information about the structures and relative energies of both rotamers is available from a variety of techniques, including (dispersed) laser-induced fluorescence,20,51,52 REMPI,19,53 and IR spectra.12,19

Information on the corresponding t/c-1-Np+ cation rotamers is provided by ZEKE,53 MATI,16 and REMPI-IR spectra.19

The results for t/c-1-Np(+) relevant for the present work can be summarized as follows.

In S0, t-1-Np is slightly more stable than c-1-Np (ΔE = 220 ± 50 cm−1)51 and its OH group is somewhat more acidic as demonstrated by the smaller O–H stretch frequency (ν1/cm−1 = 3655 vs. 3663).12

Interestingly, the energy order of both rotamers is reversed in S1E = −59 cm−1).53

In D0, t-1-Np+ is again more stable than c-1-Np+E = 495 cm−1), indicating that the potential for OH rotation is quite sensitive to both electronic excitation and ionization.53

The acidity of t-1-Np(+) is substantially increased upon ionization, as revealed by the large decrease in ν1 from 3655 to 3579 cm−1.19

No spectroscopic information is available for the acidity of c-1-Np+.

Spectroscopic and theoretical information about neutral and ionic 1-Np(+)–Ln complexes with L = Ar/N2 is rather limited.

REMPI,19,54 hole-burning,54 and IR spectra19 of t-1-Np–Ar are consistent with a π-bound structure in S0, in agreement with quantum chemical calculations.54

Similarly, the REMPI-IR spectrum of t-1-Np+–Ar in the O–H stretch range was interpreted with a π-bound isomer in D0.19

No evidence for an H-bound 1-Np(+)–Ar isomer has been reported for both S0 and D0.

Calculations predict π-bound t-1-Np–N2 to be more stable than the H-bound isomer in S0, and preliminary REMPI and hole-burning spectra appear to be consistent with this view.54

To the best of our knowledge, no spectroscopic information is available for any c-1-Np(+)–Ln complex in both S0 and D0.9,16,19,20

Such information is difficult to obtain because of the usually low population of the less stable c-1-Np rotamer in molecular beams and steric hindrance involved in the formation of H-bound c-1-Np(+)–L complexes.

Several aspects motivated the present EI-IR spectroscopic and theoretical study on size-selected 1-Np+–Ln complexes with L = Ar and N2.

(1) These clusters serve as a suitable model for solvation of the highly acidic 1-Np+ cations in a nonpolar solvent.

Particularly interesting is the competition between H-bonding and π-bonding and its consequences on the cluster growth.

As Ar and N2 are weak proton acceptors, proton transfer from 1-Np+ to the solvent may not be expected in D0 even for complete solvation.55

(2) As the ability of the OH group to form H-bonds to a ligand is correlated with its acidity, the EI-IR spectra of H-bound 1-Np+–L dimers directly probe the acidity of both 1-Np+ rotamers and eventually enable a first estimate of the unknown proton affinity of the 1-naphthoxy radical.

The detection of c-1-Np+–(N2)n in the present work corresponds to the first observation of c-1-Np+ complexes and enables thus direct comparison of both 1-Np+ rotamers.

H-bound 1-Np+–L isomers are also characterized by density functional calculations, because theoretical studies of the interaction potential in these dimers are not available.

(3) Comparison of 1-Np+–Ln with Ph+–Ln22,32–35 and H2O+–Ln56–59 reveals the influence of substitution of a mono- and bicyclic aromatic ring on the acidity of ROH+ cations.

The acidity is expected to decrease in the order H2O+ > Ph+ > 1-Np+ because of increasing charge delocalization in the aromatic ring19.


EI-IR photodissociation spectra of mass-selected 1-Np+–Ln complexes (L = Ar/N2, n ≤ 5) were recorded in a tandem quadrupole mass spectrometer (QMS1/2) coupled to an EI ion source and an octopole ion trap.50

Briefly, 1-Np+–Ln were generated in a plasma expansion created by EI of a pulsed supersonic molecular beam.

The expansion gas was produced by seeding 1-Np vapor (T ≈ 350 K) in Ar or N2 at stagnation pressures of 3–10 bar.

As described previously,33 the dominant production mechanism of 1-Np+–Ln starts with EI of 1-Np near the nozzle orifice, which is followed by three-body association in the high-pressure region of the expansion:1-Np + e → 1-Np+ + 2e1-Np+–Ln−1 + L + M → 1-Np+–Ln + M (M = L, 1-Np)This reaction sequence produces mainly the most stable 1-Np+–Ln complexes and to smaller extent less stable isomers.33–37,60,61

Alternative mechanisms involving the formation of neutral 1-Np–Ln complexes and subsequent EI play only a minor role.33

Fig. 3 shows a mass spectrum of the ion source recorded after optimization for 1-Np+–(N2)n generation.

The spectrum is dominated by Nn+ and [X–(N2)n]+ cluster ions, including X = 1-Np and the impurity H2O. The intensity ratios of 1-Np+–(N2)n are of the order of 80∶8∶2∶1 for n = 0–3, indicating the formation of weakly-bound noncovalent complexes.

The central part of the plasma expansion was extracted through a skimmer into QMS1, which was tuned to the mass of 1-Np+–Ln.

The mass-selected 1-Np+–Ln beam was then injected into an octopole ion guide, where it interacted with a tunable IR laser pulse.

Resonant vibrational excitation of 1-Np+–Ln induced the evaporation of one or more ligands: 1-Np+–Ln + → [1-Np+–Ln]* → 1-Np+–Lm + (nm)L Other photodissociation channels were not observed.

1-Np+–Lm fragments generated in the octopole were mass-selected by QMS2 and monitored as a function of the laser frequency (ν) to obtain the EI-IR spectra of 1-Np+–Ln.

Several fragment channels (m) were observed for 1-Np+–(N2)n parent complexes with n > 3.

Consequently, IR spectra were recorded simultaneously in the two dominant fragment channels.

As an example, Fig. 4 shows the mass spectrum obtained for resonant ν1 excitation of t-1-Np+–(N2)4.

Major fragment channels are m = 0 and m = 1 arising from laser-induced dissociation and m = 3 produced by metastable decay in the octopole.

To separate laser-induced fragmentation from interfering metastable decay, the ion source was triggered at twice the laser repetition rate and signals from alternating triggers (laser on, laser off) were subtracted.

As the IR spectra recorded in different fragment channels are similar, only the spectra obtained in the dominant channels are shown.

IR radiation was generated by a single-mode optical parametric oscillator laser system and calibrated to an accuracy of better than 1 cm−1 by analyzing atmospheric water absorptions along the IR laser path62.

Density functional calculations

Density functional theory (DFT) calculations were carried out for t/c-1-Np+ and their H-bound dimers at the UB3LYP/6-311G(2df,2pd) and UB3LYP/6-31G* levels of theory63 to characterize the intermolecular H-bonds and their effects on the t/c-1-Np+ properties.

As the results are qualitatively similar for both basis sets, only those obtained with the larger basis are reported.

The t/c-1-Np+(–L) properties relevant for the present work are summarized in Table 1 and Figs. 1 and 2.

For comparison, additional calculations were carried out for t/c-1-Np, Ph(+), and Ph+–L at the same theoretical level.

Table 1 includes structural and energetic attributes of the intermolecular H–L bond, such as length (RH–L), angle (θO–H–L), dissociation energy (De), and intermolecular stretch frequency (νs), as well as relevant properties of the intramolecular O–H bond, namely length (RO–H), stretch frequency (ν1), and IR intensity (I1).

In general, all coordinates were relaxed during the search for stationary points.

Intermolecular interaction energies were counter-poise corrected for basis set superposition error.64

Harmonic vibrational frequencies were scaled by a factor of 0.9516 to bring the calculated ν1 frequencies of both t-1-Np+ and Ph+ into agreement with the experimental values, ν1 = 3579 and 3534 cm−1.19,65

First, the bare t/c-1-Np(+) rotamers are considered.

Geometry optimization results in planar equilibrium structures of 1-Np in both S0 and D0, consistent with available spectroscopic data.52,53

Fig. 2 compares the potentials for internal OH rotation in 1-Np and Ph calculated for S0 and D0 including zero-point energy contributions.

In S0, t-1-Np is predicted to be more stable than c-1-Np by ΔE = 360 cm−1, in reasonable agreement with the value of ΔE = 220 ± 50 cm−1 estimated from the analysis of the UV spectrum recorded in an absorption cell.51

The barrier for internal rotation from t-1-Np toward c-1-Np, Vb = 1087 cm−1, is somewhat smaller than in Ph, Vb = 1151 cm−1.

The latter value is close to the experimental barrier determined from rotationally resolved UV spectroscopy in a molecular beam, Vb = 1215 ± 10 cm−1,66 suggesting that the employed theoretical level is of sufficient accuracy to yield reliable internal rotation potentials.

In D0, t-1-Np+ is also calculated to be more stable than c-1-Np+ and the energy difference, ΔE = 746 cm−1, is approximately twice that in S0.

The difference in stability of both rotamers in D0 and S0 corresponds to the difference in their adiabatic ionization potentials, and the predicted value is compatible with the experimental measurement, 386 vs. 281 cm−1.53

The barrier for OH rotation from t-1-Np+ toward c-1-Np+, Vb = 3198 cm−1, is significantly larger than in S0.

The corresponding barrier for Ph+ is even higher, Vb = 4514 cm−1.

The barrier height of hydroxyarenes is usually correlated with their C–O bond strength.52,66

In general, the stronger and shorter the C–O bond, the more hindered the internal rotation.

Specifically, RC–O/Å∼1.363 > 1.318 > 1.307 and Vb/cm−1 = 1087 < 3198 < 4514 for 1-Np, 1-Np+, and Ph+.

Apparently, electron donation from the OH group into the aromatic π-electron system is more efficient in D0 than in S0 and larger for Ph+ as compared to 1-Np+.

In contrast to the C–O bond, the O–H bond strength decreases along the series c-1-Np+ > t-1-Np+ > Ph+, as is signaled by the increasing bond length (RO–H/Å = 0.9666 < 0.9675 < 0.9714) and the decreasing stretch frequency (ν1/cm−1 = 3587 > 3579 > 3534).

The rise in the acidity of the OH group along this series is also mirrored by the increasing positive partial charge on the OH proton.44

In general, the differences between the t/c-1-Np+ rotamers are significantly smaller than between t/c-1-Np+ and Ph+.

Interestingly, the IR oscillator strengths for ν1 of c-1-Np+ and t-1-Np+ differ by a factor of 2.4 (I1/km mol−1 = 121 vs. 289).

The H-bound 1-Np+–L dimers feature trans-linear proton bonds of L to the acidic OH group of 1-Np+, leading to planar equilibrium geometries with Cs symmetry (Fig. 1).

Because of steric hindrance arising from the interaction with the proton on the second aromatic ring (C(8)H), the deviation from linearity is much larger for c-1-Np+–L (∼150°) as compared to t-1-Np+–L (∼170°).

The C(8)H–N and C(8)H–Ar separations of 2.71 and 3.07 Å in c-1-Np+–L are close to the values expected from van der Waals radii (RvdW/Å = 1.1, 1.5, 1.9 for H, N, Ar).67

The anisotropy of the long-range charge–quadrupole and charge–induced dipole interactions aligns the N2 ligand in such a way that the molecular axis points toward the positive charge,30,32,37,39,41,44,60,68–71 resulting in nearly linear H⋯N–N configurations in t/c-1-Np+–N2 (θH–N–N = 175/171°).

The increase in the acidity along the series c-1-Np+< t-1-Np+< Ph+ is directly correlated with the intermolecular interaction strength, leading to shorter H–L bonds (RH–L/Å = 2.59 > 2.46 > 2.35 for L = Ar and 2.11 > 2.02 > 1.95 for L = N2) with larger stretch frequencies (νs/cm−1 = 46 < 49 < 73 for L = Ar and 88 < 94 < 117 for L = N2) and higher dissociation energies (De/cm−1 = 287 < 372 < 685 for L = Ar and 1205 < 1336 < 1731 for L = N2).

The effects of H-bonding on the intramolecular O–H bonds are an elongation (ΔRO–H), a reduction in the stretch frequency (Δν1), and an enhancement in the IR oscillator strength (ΔI1).

Again, the magnitude of these effects is correlated with the H-bond strength and increases along the series c-1-Np+< t-1-Np+< Ph+: ΔRO–H/Å = 0.0016 < 0.0027 < 0.0044 for L = Ar and 0.0048 < 0.0068 < 0.0099 for L = N2; −Δν1/cm−1 = 34 < 57 < 92 for L = Ar and 91 < 134 < 195 for L = N2; ΔI1/km mol−1 = 182 < 445 < 537 for L = Ar and 453 < 977 < 1060 for L = N2.

These theoretical results demonstrate that IR spectroscopy in the O–H stretch range is a suitable tool to probe both the acidity of the t/c-1-Np+ rotamers and their ability to form H-bonds.

Similar to previous studies on (para-halogenated) Ph(H)+–Arn and Ph(H)+–(N2)n,32–35,40–42 the EI-IR spectra of the corresponding 1-Np+–Arn and 1-Np+–(N2)n complexes clearly demonstrate that the H-bonds are more stable than the π-bonds.

Hence, the energetically most favorable 1-Np+–Ln cluster growth begins with the formation of a H-bound 1-Np+–L dimer, to which further π-bound ligands are attached.

The exact position of the π-bound ligands is however not obvious (e.g., over the first or the second aromatic ring) and difficult to determine from both the present spectroscopic and theoretical approaches.

As DFT calculations do not properly describe dispersion forces, π-bound 1-Np+–L isomers were not investigated theoretically.32

On the other hand, previous studies revealed that π-bound ligands have only very minor effects on the O–H bond properties19,32,35.

IR spectra

The EI-IR spectra of 1-Np+–Arn (n ≤ 2) and 1-Np+–(N2)n (n ≤ 5) recorded in the O–H stretch range are shown in Figs. 5 and 6.

Table 2 summarizes the band maxima and widths of the ν1 transitions observed (A–C), along with their isomer assignments.

1-Np+–L Dimers

Fig. 5 compares EI-IR spectra of 1-Np+–N2 (a, b) and 1-Np+–Ar (c) with the REMPI-IR spectrum of t-1-Np+–Ar (d).19

The EI-IR spectrum of 1-Np+–Ar displays two transitions, which are attributed to the ν1 fundamentals of the H-bound (A) and π-bound (B) isomers of t-1-Np+–Ar, respectively.

These assignments are based on the band positions, relative IR intensities, and band profiles, as well as the comparison with the DFT calculations and the REMPI-IR spectrum.

The more intense band at 3538 cm−1 (A) is assigned to ν1 of H-bound t-1-Np+–Ar (Fig. 1b).

The derived complexation-induced red shift of −41 cm−1 from the corresponding t-1-Np+ transition (3579 cm−1)19 is consistent with the calculated value (−57 cm−1).

The blue-shaded band contour with a sharp P-branch head is characteristic for excitation of a proton donor stretch vibration, because the intermolecular H-bond becomes stronger and shorter in the excited state, resulting in larger rotational constants.30,32,61,70

Band B at 3580 cm−1 is assigned to ν1 of π-bound t-1-Np+–Ar, in good agreement with the REMPI-IR spectrum (Fig. 5d).19

The modest complexation-induced blue shift of +1 cm−1 and the rather symmetric band profile support this assignment, because intermolecular π-bonding has almost no influence on the O–H bond.

There are actually other binding sites, which also have little impact on the O–H bond, such as H-bonding to aromatic C–H bonds.

Although at present these alternative binding sites cannot be completely ruled out from both the experimental and the theoretical point of view, the presently favored assignment for B is to a π-bound isomer and this interpretation is used as working hypothesis for further discussion in this paper.

This scenario is supported by theoretical and spectroscopic data obtained for BzH+–Ar,44,45 Bz+–Ar,43,72–74 and PhH+–Ar,42 which indicate that intermolecular CH–Ar bonds in A(H)+–Ar are less stable than π-bonds.

The EI-IR spectrum of t-1-Np+–Ar clearly demonstrates that the H-bound isomer is the global minimum on the intermolecular potential, whereas the π-bound structure is a less stable local minimum.

Nonetheless, the H-bound isomer could not be detected in the REMPI-IR spectrum (Fig. 5d),19 because it was obtained through resonant ionization of π-bound t-1-Np–Ar.

In contrast to both t-1-Np+–Ar isomers, spectroscopic indication of the corresponding c-1-Np+–Ar dimers in the EI-IR spectra is only tentative.

The ν1 frequency of bare c-1-Np+ has not been measured,19 and the calculated value is slightly larger than for t-1-Np+ (+8 cm−1).

Some of the EI-IR spectra recorded for 1-Np+–Ar actually reveal a very weak band at 3595 cm−1 (indicated by an asterisk in Fig. 5c) appearing about 15 cm−1 to the blue of π-bound t-1-Np+–Ar.

At first glance, this signal may be attributed to ν1 of π-bound c-1-Np+–Ar, which could occur in detectable abundance only under certain experimental conditions.

If this interpretation were correct, a minor part of the blue wing of band A may arise from ν1 of H-bound c-1-Np+–Ar (Fig. 1d).

However, according to the analysis of the 1-Np+–N2 spectra (vide infra), the abundances of c-1-Np+ and its Ar complex are probably too small for observing ν1 of π-bound c-1-Np+–Ar even under favorable (e.g., warm) expansion conditions.

Thus, the interpretation of the weak 3595 cm−1 band remains open.

Fig. 5 compares EI-IR spectra of 1-Np+–N2 recorded under warm (a) and cold (b) expansion conditions, as revealed from the widths of the observed rotational band contours.

The cold spectrum (b) displays a single blue-shaded band at 3467 cm−1 (A), which is assigned to ν1 of H-bound t-1-Np+–N2 (Fig. 1a).

The measured red shift of −112 cm−1 upon H-bonding is slightly smaller than the calculated value of −134 cm−1.

Moreover, it is roughly three times larger than the one for t-1-Np+–Ar (−41 cm−1) because of the stronger H-bond.

In addition to band A, the warm 1-Np+–N2 spectrum (a) shows a transition at 3499 cm−1 (C), which is absent in the cold spectrum (b).

This band is attributed to ν1 of H-bound c-1-Np+–N2 (Fig. 1c).

The −88 cm−1 shift from calculated ν1 of c-1-Np+ (3587 cm−1) agrees with the prediction (−91 cm−1).

Both EI-IR spectra of 1-Np+–N2 do not show any absorption near 3580 cm−1, implying that the abundances of the π-bound t/c-1-Np+–N2 isomers were below the detection limit under both warm and cold conditions.

The ratios of the integrated ν1 band intensities observed in the EI-IR spectra of 1-Np+–L can be used to estimate the relative abundances of the H-bound and π-bound isomers in the plasma expansion, using the calculated ν1 IR oscillator strengths of t/c-1-Np+ and H-bound t/c-1-Np+–L (Table 1) and assuming that π-bound t/c-1-Np+–L have the same ν1 IR intensities as bare t/c-1-Np+.32,36,39

For t-1-Np+–Ar, this procedure results in an abundance ratio of NHNπ≈1.6, on the basis of the experimental ν1 intensity ratio of ≈4 and the theoretical ratio IHIπ ≈ 2.5.

For t-1-Np+–N2, a lower limit for NHNπ > 9 can be estimated from the achieved signal-to-noise ratio (≈40) and IHIπ ≈ 4.4.

The larger abundance of the H-bound t-1-Np+–L isomers suggests that they are more stable than the π-bound dimers, because the EI source produces predominantly the most stable isomer of a given complex.33,50

Moreover, the energy difference between both isomers appears to be significantly larger for t-1-Np+–N2 than for t-1-Np+–Ar, resulting in a less efficient production of the π-bound isomer for the N2 complex.

Similar to previous studies on Ph+–Ar and In+–Ar,34,37 the relative intensity ratio of the ν1 bands of H-bound and π-bound t-1-Np+–Ar depended on the expansion conditions, confirming that both transitions arise indeed from two different isomers.

The spectrum in Fig. 5c was obtained when the conditions were optimized for the production of π-bound t-1-Np+–Ar.

As the relative population of the H-bound isomer increased for decreasing effective source temperature, this isomer corresponds to the global minimum of the t-1-Np+–Ar potential, whereas the π-bound structure is a less stable local minimum.

The population ratio of the H-bound isomers of t-1-Np+–N2 and c-1-Np+–N2 in the warm spectrum of 1-Np+–N2 (Fig. 5a) is deduced as NtNc ≈ 8 from the experimental (≈18) and predicted (≈2.2) ν1 intensity ratios.

As the N2 complexation energies are similar for both 1-Np+ rotamers (1336 and 1205 cm−1), the c-1-Np+ concentration in the plasma expansion is estimated to be roughly one order smaller than that for t-1-Np+.

This result is consistent with the production mechanism described in section 2 and the internal rotation potentials in Fig. 2.

Assuming a nozzle temperature of T ≈ 350 K, the energy difference of c-1-Np and t-1-Np of ΔE ≈ 220 cm−1 corresponds to a thermal population ratio of NtNc ≈ 2..551

Adiabatic cooling in the supersonic expansion increases this ratio only slightly because both isomers are separated by a significant barrier (Vb = 1087 cm−1 corresponds to T ≈ 1560 K).75

For example, NtNc ≈ 8 was observed in a previous molecular beam experiment.20

Assuming similar cooling of the t/c-1-Np rotamers in our expansion and mainly vertical ionization of the EI process, eqn. (1a), a similar ratio is obtained for the ionic t/c-1-Np+ rotamers because isomerization is strongly hindered by a large barrier in D0 (Vb = 3198 cm−1).

Owing to comparable complexation energies for H-bonding and also π-bonding for both 1-Np+ rotamers, the t/c-1-Np+–L dimer population ratios should be similar to the t/c-1-Np+ monomer ratios, eqn. (1b), i.e.NtNc ≈ 3–10 depending on the efficiency of adiabatic cooling of neutral t/c-1-Np.

Thus, the deduced ratio of NtNc ≈ 8 for the H-bound t/c-1-Np+–N2 isomers is within the expected range.

If the weak 3595 cm−1 band in the 1-Np+–Ar spectrum were indeed due to ν1 of π-bound c-1-Np+–Ar, the relative abundance of π-bound t/c-1-Np+–Ar (and also t/c-1–Np+) would be derived as NtNc ≈ 2.1 from the experimental (≈5) and predicted (≈2.4) ν1 intensity ratios.

This rather small ratio would imply an unexpectedly high population of both c-1-Np+ and π-bound c-1-Np+–Ar and makes this tentative assignment questionable.

Larger 1-Np+–Ln complexes

Fig. 6 reproduces the EI-IR spectra of 1-Np+–Arn (n ≤ 2) and 1-Np+–(N2)n (n ≤ 5) recorded in the dominant fragment channels (Table 3).

The 1-Np+–Ar spectrum (a) is dominated by two transitions at 3538 (A) and 3580 (B) cm−1 assigned to ν1 of H-bound and π-bound t-1-Np+–Ar (section 4.1).

The 1-Np+–Ar2 spectrum (b) reveals only a single band at 3539 cm−1 (A), which is shifted by merely +1 cm−1 from band A of H-bound t-1-Np+–Ar.

Consequently, this band is assigned to ν1 of a t-1-Np+–Ar2 isomer, which is obtained by adding a π-bound ligand to H-bound t-1-Np+–Ar, denoted t-1-Np+–Ar2(H/π).

A blue shift of +1 cm−1 is also observed for attachment of a π-bound Ar ligand to bare t-1-Np+.

The ν1 transitions of t-1-Np+–Ar2 isomers with two π-bound ligands, t-1-Np+–Ar2(2π), are expected near 3580 cm−1 (B).

As the 1-Np+–Ar2 spectrum lacks any signal in that frequency range, the abundance of t-1-Np+–Ar2(2π) isomers is below the detection limit.

The abundance ratio of t-1-Np+–Ar2(2π) and t-1-Np+–Ar2(H/π) is estimated to be less than 5%.

This result confirms that the H-bond between Ar and t-1-Np+ is stronger than the π-bond.

Hence, the preferred solvation sequence in small t-1-Np+–Arn complexes begins with the formation of an H-bound t-1-Np+–Ar dimer core, which is further solvated by (n − 1) π-bound ligands.

The 1-Np+–N2 spectrum (Fig. 6c) displays two transitions at 3467 (A) and 3499 (C) cm−1 assigned to ν1 of the H-bound isomers of t-1-Np+–N2 and c-1-Np+–N2 (section 4.1).

Attachment of four further N2 ligands causes only small incremental blue shifts, suggesting that the observed t/c-1-Np+–(N2)n complexes have one H-bound and (n−1) π-bound ligands.

Fig. 7 visualizes the ν1 frequencies of the most stable isomers of t-1-Np+–Arn, t-1-Np+–(N2)n, and c-1-Np+–(N2)n as a function of the cluster size n.

Table 3 lists the photofragmentation branching ratios measured for resonant ν1 excitation of the H/(n − 1)π isomers of t-1-Np+–Arn and t-1-Np+–(N2)n.

In agreement with previous studies on related systems,33,37,39,42–44,50,57,60,61 the range of photo-induced fragment channels (m) for a given parent cluster size (n) is rather narrow (Fig. 4).

This information is used to roughly estimate ligand binding energies within the framework of a simple model, which assumes that the absorbed photon energy (ν1) is available for subsequent ligand evaporation.33

The ligands are classified in H-bound and π-bound ones with dissociation energies D0(H) > D0(π), and all π-bound ligands are assumed to have the same binding energy.

Larger t-1-Np+–(N2)n complexes evaporate on average about five π-bound ligands upon ν1 excitation (∼3500 cm−1), yielding an estimated dissociation energy of the order of D0(π) ∼ 650 ± 150 cm−1.

Using this information, the branching ratios observed for n = 4 and 5 lead to 500 < D0(H)/cm−1 < 2000, i.e.D0(H) ∼ 1250 ± 750 cm−1, which agrees with the predicted value, De(H) = 1336 cm−1.

Within the cluster size range investigated, the ν1 bands assigned to c-1-Np+–(N2)n display photofragmentation branchings similar to the ν1 bands of t-1-Np+–(N2)n, confirming that D0(π) and D0(H) are roughly comparable for both 1-Np+ rotamers.

The t-1-Np+–Ar2(H/π) trimer evaporates both ligands upon ν1 excitation, implying that D0(H) + D0(π) < 3540 cm−1.

Further discussion

The EI-IR spectra of 1-Np+–Arn are dominated by ν1 absorptions assigned to Ar complexes of the more abundant t-1-Np+ rotamer.

Moreover, the relative ν1 intensities of the H-bound and π-bound dimers suggest that the H-bond of Ar to t-1-Np+ is more stable than the π-bond, D0(H) > D0(π).

This conclusion is in line with the lack of detection of the 1-Np+–Ar2(2π) isomer.

The binding energies of the π-bound dimers of Bz+–Ar, para-difluorobenzene+–Ar, In+–Ar, and Ph+–Ar were accurately measured as 512 ± 3, 572 ± 6, 537 ± 10, and 535 ± 3 cm−1, respectively.15,23,76,77

As the strength of the π-bond of Ar to an aromatic cation A+ is apparently rather insensitive to the detailed structure of A+,44D0(π) of t/c-1-Np+–Ar may be estimated as ∼550 ± 50 cm−1.

(This is contrast to neutral A–Ar dimers, where D0(π) increases with the number of aromatic rings15.) The t-1-Np+–Ar2 photofragmentation data show that D0(H) + D0(π) < 3540 cm−1, yielding 500 < D0(H)/cm−1 < 3040.

The upper limit can be further reduced because D0(H) of t/c-1-Np+–Ar should be somewhat lower than D0(H) of Ph+–Ar (670 ± 140 cm−1).33

First, Δν1 of H-bound t-1-Np+–Ar is much smaller than Δν1 of H-bound Ph+–Ar (1.15 vs. 2.0%),33 because t-1-Np+ is less acidic than Ph+.

Second, the NHNπ ratio of t-1-Np+–Ar in the EI source is lower than for Ph+–Ar under comparable conditions.

When the conditions are optimized for the generation of the π-bound isomers, NHNπ ≈ 1.6 for t-1-Np+–Ar and ≈4 for Ph+–Ar.35

This comparison suggests that D0(H) − D0(π) is larger for Ph+–Ar than for t-1-Np+–Ar.

Because D0(π) of Ph+−Ar and t-1-Np+–Ar are comparable, D0(H) of Ph+–Ar provides an upper limit for D0(H) of t-1-Np+–Ar, resulting in 500 < D0(H)/cm−1 < 800 or D0(H) ≈ 650 ± 150 cm−1 for t-1-Np+–Ar.

Thus, the calculated dissociation energy of De = 372 cm−1 substantially underestimates the true interaction energy, in line with conclusions previously drawn for UB3LYP/6-311G(2df,2pd) calculations of related Ar complexes.32,35,36,41

Surprisingly, despite the underestimated interaction, this theoretical level slightly overestimates the Δν1 shifts.

In the limit of adiabatic separation of inter- and intramolecular degrees of freedom, the Δν1 shifts correspond to the changes in the intermolecular binding energies upon ν1 excitation.

The −41 cm−1 shift for H-bound t-1-Np+–Ar implies that the interaction increases by 5–8%, resulting in D0(H) ≈ 690 ± 150 cm−1 in the v1 = 1 state.

On the other hand, the +1 cm−1 shift for π-bound t-1-Np+–Ar means that ν1 excitation only slightly destabilizes the intermolecular π-bond (∼550 ± 50 cm−1) by less than 0.3%.

Available REMPI,19,54 hole-burning,54 and IR spectra19 of t-1-Np–Ar have been interpreted with a π-bound global minimum structure.

This view is supported by recent quantum chemical calculations at the ri-MP2/ccpVTZ level, which predict the π-bond to be more stable than the H-bond in S0.54

No experimental evidence for the less stable H-bound isomer has been reported so far.19,54

The π-bound isomer optimizes the dispersion forces between Ar and the π-electron system of the aromatic ring, which dominate the attractive part of the intermolecular potential in S0.

On the other hand, the EI-IR spectrum of t-1-Np+–Ar clearly shows that the H-bond is the preferred intermolecular recognition site in D0, because of the additional induction forces between the charge distribution in t-1-Np+ and the polarisability of Ar.

In particular, the large positive partial charge on the acidic O–H proton causes the substantial additional stabilization of the H-bond on ionization.44

The ionization-induced switch in the preferred recognition motif from π-bonding to H-bonding has now been demonstrated for several A–Ar and A–CH4 dimers involving aromatic molecules A with acidic functional YHk groups (Y = O, N) and appears to be a general phenomenon.

Investigated molecules include A = (para-halogenated) Ph,32–35 An,36 In,37 and 1-Np.

H-bonds to Ar are also the most stable binding pattern for protonated aromatic molecules (AH+) featuring acidic functional groups, such as PhH+ or 1-NpH+.40–42,78

Significantly, Ar and CH4 prefer intermolecular π-bonds over H-bonds to A(H)+ without acidic substituents, such as Bz(H)+.43–45

Aromatic and aliphatic C–H bonds in A(H)+ are only little acidic, so that dispersion forces (favoring π-bonds) override the induction forces (favoring H-bonds).

The drastic change in the intermolecular potential and particularly in the geometry of the most stable structure of 1-Np–Ar induced by ionization explains why the H-bound t-1-Np+–Ar isomer has completely escaped detection in the REMPI-IR spectrum of this cation dimer (Fig. 5d).19

In this experiment, t-1-Np+–Ar dimers were prepared by REMPI of π-bound t-1-Np–Ar via the intermediate S1 state.

Consequently, the Franck–Condon factors for populating the H-bound global minimum of t-1-Np+–Ar are nearly vanishing.

This example demonstrates the severe limitations of photoionization techniques for the characterization of the most stable isomers of cluster cations in cases where the cluster structure changes drastically upon ionization.

Such significant structural changes are, however, typical for ionization of a large variety of complexes and the rule rather than the exception.31,48,49

Similar to Ph(H)+–N2,32,33,41,42 An+–N2,39 and In+–N2,37 the H-bound t/c-Np+–N2 isomers correspond to the global minima on their intermolecular potentials.

The lack of absorptions of π-bound t/c-Np+–N2 in the EI-IR spectra demonstrates that these are substantially less stable local minima.32,39

Similarly, π-bound isomers were not observed in the EI-IR spectra of An+–N2 and Ph(H)+–N2.32,39,41

In contrast, π-bound In+–N2 could be weakly observed suggesting that π-bonding becomes competitive with H-bonding for A(H)+–N2 dimers with less acidic A(H)+.

In line with this trend, N2 complexes of A(H)+ without acidic functional groups, such as Bz(H)+, prefer π-bonds with N2 over H-bonds.43–45

The photofragmentation data of 1-Np+–(N2)n yield a binding energy of the order of D0(π) ∼ 650 cm−1 for both 1-Np+ rotamers.

This value is similar to the dissociation energies of other π-bound A(H)+–N2 complexes, such as Ph+–N2 (D0 = 750 ± 150 cm−1),33 An+–N2 (D0 = 700 ± 200 cm−1),39 BzH+–N2 (D0 ∼ 800 cm−1),44 and the carbenium isomers of PhH+–N2 (D0 = 750 ± 150 cm−1).42

Similar to the corresponding Ar complexes, the dissociation energies of π-bound A(H)+–N2 are relatively insensitive to the detailed structure of A(H)+.44

This is in contrast to the strength of the H-bonds in A(H)+–N2, which depends sensitively on the acidity of the proton donor group of A(H)+.

Photofragmentation data for 1-Np+–(N2)n yield D0(H) = 1250 ± 750 cm−1 for both 1-Np+ rotamers.

According to the ν1 band shifts and the calculations, t-1-Np+ is somewhat more acidic than c-1-Np+, resulting in slightly stronger H-bonds.

However, D0(H) of both t/c-1-Np+–N2 dimers should be significantly smaller than D0(H) of Ph+–N2, which was measured as 1640 ± 10 cm−1.38

The Δν1 shifts of −112 and −88 cm−1 correspond to the increases of the H-bond strengths in t-1-Np+–N2 and c-1-Np+–N2 upon ν1 excitation.

Presently, it is not established which type of intermolecular bond is favored by 1-Np-N2 in S0. ri-MP2/ccpVTZ calculations for t-1-Np–N2 predict the π-bond to be more stable than the H-bond.54

REMPI and hole-burning spectra suggest that only one isomer is produced in the supersonic expansion and that this isomer is probably π-bound.54

Interestingly, Ph–N2 prefers H-bonding,22,26,38 whereas N2 dimers of less acidic An and Bz have π-bound global minima in S0.27–29

Apparently, the most favorable recognition motif in neutral A–N2 dimers is determined by a subtle balance between dispersion, induction, and electrostatic forces, which is governed by the polarisability and permanent multipole moments of A as well as the acidity of the YHk group (if present).

In contrast, the EI-IR spectra of t/c-1-Np+–N2 as well as Ph(H)+–N2,32,33,41,42 An+–N2,39 and In+–N2,37 clearly show that H-bonding is the stronger interaction type for the cation.

This result suggests that this trend is quite common for A(H)+–N2 dimers involving acidic A(H)+ ions.

Similar to A(H)+–Ar, A(H)+–N2 without acidic groups, such as Bz(H)+–N2, feature intermolecular π-bonds in the cation ground state.43–45

On the other hand, when the number of aromatic π-electrons and thus the dispersion forces are reduced to a minimum, such as in cyclic C3H3+ with only two π-electrons, N2 actually prefers H-bonding to the aromatic CH protons over π-bonding.60,69,71

The optimal 1-Np+–L interaction is for any angular orientation significantly stronger than the L–L attraction, De ∼ 100 cm−1 for both Ar2 and (N2)279,80 Thus, the 1-Np+–Ln cluster growth is mainly driven by the 1-Np+–L dimer potential, because three-body forces are small for ion complexes with nonpolar ligands.

Consequently, the solvation sequence for the most stable 1-Np+–Ln complexes starts with the formation of H-bound 1-Np+–L, which is further solvated by (n − 1) π-bound ligands.

A similar cluster growth was deduced for Ph(H)+–Ln and An+–Ln with L = Ar and N2, which also starts with the solvation of the acidic protons of the OH(2) and NH2 groups by H-bound ligands, before further π-bound ligands are attached to the aromatic ring.33,36,39,40,42

Fig. 7 plots the ν1 frequencies of the most stable isomers for a variety of A(H)+−Arn (a) and A(H)+–(N2)n (b) complexes as a function of the cluster size n.

The A(H)+ ions include monohydroxyarene radical cations (A+ = Ph+, t-1-Np+, c-1-Np+) as well as carbenium ions of PhH+, which all possess a single OH group.

The plots mirror the preferred evolution of the solvation (sub)shells in these clusters, denoted H/(n − 1)π.

The first H-bound ligand induces a large ν1 red shift because of the destabilization of the O–H bond upon intermolecular H-bonding.

The size of Δν1 is correlated with the strength of the intermolecular interaction.

Thus, |Δν1| is larger for N2 dimers than for Ar dimers and increases for rising OH acidity in the order c-1-Np+< t-1-Np+ < PhH+ < Ph+, which is also reflected by decreasing ν1 frequencies of the bare monomers (n = 0).

The H-bound dimers (n = 1) are further solvated by π-bound ligands, which induce small incremental blue shifts of ν1.

Thus, π-bound ligands slightly destabilize the intermolecular H-bond to the first ligand via noncooperative three-body forces, which in turn stabilize the intramolecular O–H bond.

As expected, the ν1 frequencies are not converged at the largest cluster size investigated (n ≤ 7), because the first solvent shell around A(H)+ is not complete yet.

The ν1 shift for t-Np+–(N2)5 of −77 cm−1 amounts to 2.15%.

Unfortunately, neither Ar nor N2 matrix isolation studies are available for any of these aromatic A(H)+ ions,81 preventing comparison of the A(H)+–Ln cluster band shifts with the bulk limit (n → ∞).

The complexation-induced red shift in the proton donor stretch vibration, ΔνX–H, of H-bound X–H+–L dimers is correlated with the difference in the proton affinities (PA) of the two bases X and L.18,30,33,70,82

The smaller PA(X)–PA(L), the stronger the intermolecular H–L bond and the larger ΔνX–H.

This relation may be used to estimate unknown PA values of bases X from IR spectra of their X–H+–L dimers.

Recently, this procedure was applied to XH+ = In+ to obtain the first experimental determination of the PA of the indolyl radical.37

Fig. 8 plots the relative red shifts |ΔνX–H|/νX–H as a function of PA(X) for a series of H-bound XH+–N2 and XH+–Ar dimers, including XH+ = SiOH+, Ph+, In+, An+, and t-1-Np+ (Table 4).

The shifts are larger for L = N2 compared to L = Ar, because PA(N2) > PA(Ar) (494 > 369 kJ mol−1).83

For a given L, the shifts decrease in the order XH+ = SiOH+ > Ph+ > t-1-Np+ > In+ > An+ because of increasing PA(X).

The PA of the t-1-naphthoxy radical has not been measured so far.

Linear interpolation from the known PA values of the anilino and phenoxy radicals yields PA ≈ 908 ± 5 kJ mol−1 for t-1-naphthoxy (Fig. 8).

As the uncertainty of this method for determining PA values is not well documented, the error is enlarged to ±30 kJ mol−1, yielding 908 ± 30 kJ mol−1.

The PA of c-1-naphthoxy is predicted to be larger by around 10 kJ mol−1 according to the smaller Δν1 values.

This example demonstrates that IR spectroscopy of cluster ions can be used to selectively probe thermochemical properties, such as the PA, of specific conformers of transient radicals.

The attraction in weakly H-bound XH+–L dimers arises mainly from electrostatic and inductive forces.30,56

In general, the H-bond strength correlates with the positive partial charge located on the intermediate proton, qH.44

The larger qH, the more acidic the OH group and the larger the donation of electron density into the aromatic ring.

Because delocalization of the positive charge is more pronounced for 1-Np+ than for Ph+, as also indicated by the lower ionization potential, the donation of electron density from the OH group to the ring is smaller for 1-Np+.19

Consequently, the dissociation energies of H-bound 1-Np+–L are systematically lower than those of Ph+–L.

This trend is not only observed for nonpolar ligands, such as Ar and N2 (Table 1), but also for polar ligands, such as H2O: D0(H)/cm−1 = 6520 ± 50 and 5547 ± 75 for Ph+–H2O and t-1-Np+–H2O, respectively.15–17

On the other hand, t-1-Np appears to be slightly more acidic than Ph in S0, as is indicated by the slightly stronger H-bond to H2O: D0(H)/cm−1 = 2035 ± 69 and 1916 ± 50 for t-1-Np–H2O and Ph–H2O, respectively.9,15

Spectroscopic and theoretical data demonstrate that neutral 1-Np is a much stronger H-bond donor than H2O. For example, the dissociation energy of t-1-Np–H2O (D0 = 2035 ± 69 cm−1)9 is approximately twice that of H2O–H2O (D0 ∼ 1200 cm−1),84,85 leading to a larger red shift of the proton donor stretch frequency, −148 vs. −56 cm−1.12,86

The reversed situation is observed for the cationic species, for which H2O+ is a much better H-bond donor than 1-Np+.

For example, the dissociation energy of H-bound H2O+–Ar is much larger than that of 1-Np+–Ar, D0 ∼ 2100 ± 700 vs. 650 ± 150 cm−1, accompanied by larger frequency shifts, Δν1 = −541 vs. −41 cm−1.56,57

The delocalization of the positive charge in the aromatic π-electron system implies that the OH group in 1-Np+ is much less acidic than those of H2O+.

In conclusion, the acidity of neutral ROH increases along the series H2O < Ph < 1-Np, whereas for the ROH+ cations the trend is reversed, H2O+ > Ph+ > 1-Np+.

The Δν1 shifts of A(H)+–Arn and A(H)+–(N2)n as a function of n (Fig. 7) demonstrate that the acidity of the OH group strongly depends on both the solvent type and the degree of solvation.

Asymmetric solvation of A(H)+ by the weak proton acceptors Ar and N2 in the first solvation shell indicates that the acidity of the OH group is most enhanced for the H-bound dimers, because ν1 is smallest for n = 1.

Further solvation with π-bound ligands reduces the acidity again.

This effect is typical for interior ion solvation by surrounding polar and nonpolar ligands, because three-body polarization forces are noncooperative.50,56,87–90

Thus, even in the limit n → ∞, proton transfer from A(H)+ to the solvent is not expected to occur for these A(H)+–Arn and A(H)+–(N2)n complexes.55

In contrast, for strong proton acceptors, such as L = H2O and NH3, which are able to form H-bonded solvent networks, the PA of the Ln cluster increases drastically with n.

Thus, for such A(H)+–Ln complexes, the H-bond between A(H)+ and the Ln network becomes stronger as n increases, because three-body polarization forces are large and cooperative.88,89

Consequently, proton transfer from A(H)+ to Ln can be observed for complexes larger than a critical size.

For example, proton transfer occurs in Ph+–(H2O)n for n > 2 8 and in Bz+–(H2O)n for n > .388,89

At present, it is not established how many H2O ligands are required to induce proton transfer in t/c-Np+–(H2O)n.

ZEKE and MATI spectra of t-1-Np+–H2O show that the proton is not transferred for n = .116,17

As both 1-Np+ rotamers are less acidic than Ph+, it is expected that at least n > 2 is required for t/c-Np+–(H2O)n.

Concluding remarks

The acidity of the t/c-1-Np+ rotamers and their microsolvation in nonpolar hydrophobic solvents have been investigated by DFT calculations and IR spectra of size-selected 1-Np+–Arn (n ≤ 2) and 1-Np+–(N2)n (n ≤ 5) complexes.

Analysis of the n- and L-dependent complexation-induced frequency shifts of the O–H stretch vibration (Δν1) and photofragmentation branching ratios of 1-Np+–Ln provides information about the microsolvation of both 1-Np+ rotamers in Ar and N2.

The IR spectra demonstrate that the preferred ion–ligand binding motif between 1-Np+ and Ar/N2 is H-bonding to the acidic OH group, whereas π-bonding to the aromatic ring is less favorable.

Consequently, the preferred 1-Np+–Ln cluster growth begins with the formation of H-bound 1-Np+–L dimers, which are further solvated by (n − 1) π-bound ligands.

In general, the H/π-bonds in 1-Np+–(N2)n are stronger than those in 1-Np+–Arn, mainly because of the additional charge–quadrupole interaction in the N2 complexes. t-1-Np+ is found to be slightly more acidic than c-1-Np+ but both 1-Np+ rotamers are considerably less acidic than Ph+.

Increasing charge delocalization causes the acidity of ROH+ cations to decrease along the order H2O+ > Ph+ > 1-Np+, a trend opposite to the one observed for the corresponding neutral ROH molecules.

The Δν1 shifts of H-bound t-1-Np+–L yield a first experimental estimate for the proton affinity of the t-1-naphoxy radical as ∼908 ± 30 kJ mol−1, demonstrating that IR spectroscopy of cluster ions can be used to probe thermochemical properties of transient radicals.

The most stable 1-Np+–Ar structure (H-bound) differs qualitatively from that of the neutral dimer (π-bound), emphasizing the large impact of ionization on the interaction potential and the preferred recognition motif between aromatic molecules and nonpolar ligands.

The ionization-induced switch in the preferred binding type in A(+)–Ar from π-bonding to H-bonding has now been established for a large variety of A(+) molecules with acidic functional YHk groups (Y = O, N) and seems to be a general phenomenon.

Significantly, the t/c-1-Np+–Ln complexes were generated in an EI cluster ion source, which predominantly produces the most stable structure of a given cluster ion.

The global minimum structure found for t-1-Np+–Ar (H-bound) differs from the geometry observed in photoionization spectra (π-bound), demonstrating that the EI source is more generally applicable than photoionization for the spectroscopic characterization of global minima of cation complexes.